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36                PRACTICAL  ORGANIC  CHEMISTRY

as crystals of the solvent begin to separate the mercury thread
will shoot up. Keep stirring occasionally and tapping the
thermometer, and read off the maximum point reached by means
of a lens. This gives a rough indication of the freezing-point
of the benzene. Take out the inner tube and melt the crystals
by warming the tube in the hand, and replace it in the apparatus.
Repeat the experiment, cooling the solvent not more than 0*2°
below its freezing-point before stirring. Make two or three
determinations in this way. The results should not differ by
more than o'or. Fuse some naphthalene in a basin and break
it up into small lumps or mould into pellets (p. 39). Weigh a
piece of about 0*1 to 0*2 gram on a watch-glass. Raise the cork
of the inner tube and drop the naphthalene in. Let it dissolve
and then determine the freezing-point of the benzene as before.
Repeat the process by dropping one or two fresh pieces of
naphthalene into the same solvent. At the end of the operation
remove the thermometer and stirrer, and weigh the benzene in
the inner tube with the cork. After deducting the weight of
naphthalene, the weight of the benzene will be approximately
the mean of the first and final weighings.

Example.—Using the same solvent and adding successively
three lots of substance (naphthalene), the following results were
obtained :—

W.              a.              M.        Mean.

0-0985
0-0729
0-1193

97

0-403
0-305
0-486

126

123-2

126-8

Calculated for ClftHR; M = 128.

In determining the molecular weight of liquids the apparatus
shown in Fig. 82 (p. 210) is convenient for weighing and trans-
ferring the liquid to the tube.
The Eykman Depressimeter.—For rapid but less
accurate determinations the apparatus of Eykman may be used,
which is shown in Fig. 31. It consists of a small vessel, into
the neck of which a thermometer is ground. The thermometer
is of the Beckmann type but divided into twentieths of degrees.
Phenol, m.p. 42*5°, is usually employed as the solvent. The
vessel and thermometer are dried and weighed. Phenol melted
on the water-bath is poured in to within about 5 c.c. of the neck,